Like the rest of the developed world, the birth rate in the U.S. has been falling for many years: in 2012, it was measured at an all-time low of 63.0 per 1000 women aged between 15 and 44. Without immigration, the birth rate alone would not be enough to replenish the U.S. population, and in many developed countries the population is shrinking. For example, Japan is on track to return to its 1950s-era population by 2050 or so, and from there it will continue to fall dramatically.

Liberals tend to celebrate these trends, or at least tacitly endorse them. Though overpopulation isn’t the huge bugaboo it was back in the '70s, it’s still generally regarded as a problem, not least because (so it is argued) a huge number of humans will naturally lead to a ruinous overtaxing of the Earth's resources. But there are two problems with this view.

First: all things being equal, a low birth rate means greater inequality of wealth. Why? Because it means inheritances are more concentrated. Conversely, if there are more children, inheritances are more spread out. The economist Thomas Piketty — whose new book Capital in the Twenty-First Century shows how inheritance has historically driven income inequality — made this point at a talk this week:

In a number of countries we are moving in the direction of declining population, and this is quite frightening. I just want to emphasize...that has consequences for the structure of inequality. If each family has only one kid, then you inherit from both sides...

Right now in countries like Germany, Japan, and China, the size of the generations that are born right now are 30 percent smaller than generation of their parents...

Potentially this could make the relative importance of inherited wealth even higher than in 19th-century Europe. [Speed Reads]

Second: total population is not the most important factor affecting climate change and the environment. Quite often, I hear lefty friends of mine say that there are just "too many people," often coupled with some grim joke about a mass die-off. (There are even human extinction movements.) But this is mistaken. The most important factor with regards to climate change is the structure of society’s resource use, which dwarfs the effects of of raw population.

Let’s consider 2010 carbon dioxide emissions per capita for a few countries, courtesy of the World Bank (figures are in metric tons):

Burkina Faso: 0.1

Colombia: 1.6

Austria: 8.0

Finland: 11.5

France: 5.6

Kuwait: 31.3

United States: 17.6

Kuwait has a per-person carbon emission rate that is 313 times greater than Burkina Faso's. That’s an extreme example, and driven mostly by the fact that Burkina Faso is very poor, but the point is clear. More pertinently, consider the difference between France and the United States. The average American creates more than three times as much carbon pollution as the average French person, which means that if we reduced American emissions to French levels, we could then double our current population and still be far below our current aggregate emissions level.

In other words, the whole world could stop having babies tomorrow, and there would be still be more than enough residual population to cause catastrophic climate change before humanity goes extinct.

I've only discussed carbon emissions, but similar arguments hold for land use (in which density far outranks raw population in effects) and raw materials (in which wealth, efficiency, and recycling are the important factors, not population).

That’s not to say that overpopulation is a non-issue. Especially for poorer countries stuck in a subsistence agriculture trap — in which agricultural land is divided among very many low-producitivity smallholders — it can be a serious problem. But for rich countries, making society more efficient is far more important than controlling population growth when it comes to the environment.

And as Piketty suggests, for countries with a declining population, bringing populations back up to a stable level could be a critical anti-plutocracy initiative — one that could presage the kind of climate change legislation that has been opposed by wealthy corporations.